Centrifugation-assisted preparation of additive-free carbon-decorated magnetite electrodes

ABSTRACT

Iron nanoparticles that are useful for constructing electrodes for lithium ion batteries and a method of making said particles is disclosed herein. The nanoparticles may include magnetite. The electrode may be constructed by centrifuging the nanoparticles to a current collector, such as a disc of copper, without the use of an extrinsic binder. The solvothermal method of making nanoparticles decreases the time of the procedure from about 24 hours to about 75 minutes. The method of making electrode decreases the complexity and number of steps compared to the conventional procedure to prepare an electrode, and eliminates the use of additives (binder and current enhancer) and toxic NMP solvents in the electrode preparation process.

RELATED APPLICATIONS

The present patent document claims the benefit of the filing date under35 U.S.C. § 119(e) of Provisional U.S. Patent Application Ser. No.62/065,274, entitled “Centrifugation-Assisted Preparation of AdditiveFree Carbon Doped Magnetite Electrodes,” filed Oct. 17, 2014, which ishereby incorporated by reference.

BACKGROUND

Rechargeable lithium-ion batteries (LIBs) have been attracting muchattention in the past few decades. LIBs with high power and energydensity are highly desirable in order to meet the increasing demand forenergy storage, in particular, electric vehicles. Currently, LIBsexclusively use carbon as negative-electrode materials for its goodcycling performance based on the intercalation/de-intercalationmechanism of Li storage. However, the theoretical capacity of graphiteat 372 mA h g⁻¹ (based on LiC6) has almost been achieved.

On the other hand, the intercalation of lithium into graphite mainlyoccurs at low potential close to zero V (vs. Li/Li+). Accidentalovercharge at high currents may lead to the possible formation oflithium dendrites that short circuit anode and cathode and cause thermalrunaway or even a fire. Recently, much attention has been devoted to thedevelopment of carbon-alternative negative-electrode materials, whichmust have higher specific capacity and better safety performance thanthe widely adopted carbon anode.

Various metal oxides have been extensively explored ascarbon-alternatives, in particular, magnetite Fe₃O₄. Fe₃O₄ has atheoretical capacity of 927 mA h g⁻¹ and its potential of lithiuminsertion based on the conversion-type mechanism is significantly highercompared to that of carbon. Other advantages are low cost, abundance,environmental friendliness, and especially the high electricalconductivity at room temperature of about 2.5×10² S cm⁻¹ among all metaloxides.

High electrical conductivity is rarely observed in other metal oxidesinvestigated for application in LIBs (e.g. α-Fe₂O₃ has an electricalconductivity of rv10⁻⁴ S cm⁻¹, which is six orders of magnitude or ×10⁻⁶lower than magnetite). High conductivity is highly desirable forelectrodes in LIBs to facilitate charge transfer. However, as one of theconversion-type negative-electrode materials, the volume expansion(˜200%) of magnetite is much larger than that of insertion-typenegative-electrode materials (such graphite) upon lithium insertion.This huge volume variation or pulverization could cause disintegrationof the electrode and lead to poor cycling performance.

This poor cyclability becomes one of the obstacles to commercializeFe₃O₄ as negative-electrode materials in LIBs. On the other hand, basedon the conversion-type lithium storage mechanism, metallic iron (Fe⁰)nanograins will be generated through electrochemical reduction. Fe⁰nanograins are highly reactive toward the organic electrolyte. Theirreversible reactions on the surface of Fe⁰ nanograins with theelectrolyte could also cause poor cycling performance. To address thepoor cycling performance of Fe₃O₄, one strategy is to adopt nanoscalematerials to buffer the volume variation during the charge-dischargeprocess. The other strategy is to add or coat with carbon to minimizethe exposure between Fe⁰ nanograins and organic electrolyte as well asto increase the electrical conductivity. For example, Fe₃O₄-carboncomposites have been demonstrated to achieve a certain level of successin terms of electrochemical performances. The composites includeFe₃O₄-carbon nanospindles, Fe₃O₄-carbon nanorings, and C-encapsulatedFe₃O₄ nanoparticles homogeneously embedded in porous graphitic carbonnanosheets. Also reported have been a series of nanostructured ironoxide based anode materials for LIBs, such as carbon coated Fe₂O₃nanorods, nanocubes, microboxes, nanotubes, nanodiscs, nanospheres,hollow microspheres of Fe₃O₄, nanohorns on CNTs, and Fe₃O₄ nanosphereswith carbon matrix. Therefore, Fe₃O₄ could find promising application asnegative electrodes in LIBs.

Moreover, manufacturing processes for electrodes for LIBs currently takemultiple steps including mixing, roll-coating, compressing, and dryingand involve the use of organic chemicals, some of which can be toxic, asbinders and solvents.

It has been a challenge to develop nanoparticles that allow forexploitation of the advantageous properties of iron compounds for use inlithium ion batteries, as well as methods for decreasing the complexityand time of manufacture for electrodes.

BRIEF SUMMARY

In one embodiment, the disclosure provides an electrode for a lithiumion battery. The electrode includes a metal current collector having afirst face and a second face opposite the first face; and a nanoparticlecomprising an iron oxide compound and carbon, the nanoparticle beingdisposed on the first face of the metal current collector. Thenanoparticles may be porous rather than solid.

In another embodiment, this disclosure provides a method of making ananoparticle. The method includes combining an iron source with a carbonsource in an aqueous solution defining an iron-carbon solution;combining the iron-carbon solution with a porosity-increasing compoundand an alcohol to create a mixture; and heating the mixture at apredetermined temperature for a predetermined amount of time to form ananoparticle.

In another embodiment, the present disclosure provides a nanoparticlecomprising a porous composite of α-Fe₂O₃ and carbon, the nanoparticlehaving a substantially ellipsoid structure.

The invention will be further described, in non-limiting fashion, in thedetailed description below and in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic representation of a step of acentrifugation-assisted process (CAP) for making an additive-freeelectrode;

FIG. 1B is a schematic representation of another step of acentrifugation-assisted process for making an additive-free electrode;

FIG. 1C is a schematic representation of another step of acentrifugation-assisted process for making an additive-free electrode;

FIG. 1D is a view of the first faces of current collectors mentioned inthe procedures of FIGS. 1A-1C;

FIG. 1E is a view of the second faces opposite the first faces of thecurrent collectors of FIG. 1D;

FIG. 1F is a schematic representation of a centrifugation-assistedprocess for making an additive-free electrode;

FIG. 2A is an x-ray diffraction (XRD) pattern characterizing anolive-like porous Fe₂O₃/C nanoparticle;

FIG. 2B is a field emission scanning electron microscope (FESEM) imageof Fe₂O₃/C nanoparticles;

FIG. 2C is a field emission scanning electron microscope (FESEM) imageof Fe₂O₃/C nanoparticles;

FIG. 2D is a field emission scanning electron microscope (FESEM) imageof Fe₂O₃/C nanoparticles;

FIG. 2E is a transmission electron microscopy (TEM) image of Fe₂O₃/Cnanoparticles;

FIG. 2F is a transmission electron microscopy (TEM) image of Fe₂O₃/Cnanoparticles;

FIG. 2G is an energy-dispersive X-ray spectroscopy (EDS) analysis of anolive-like Fe₂O₃/C nanoparticle;

FIG. 3A is a FESEM image of porous Fe₂O₃/C nanoparticles in accordancewith another embodiment of the present disclosure;

FIG. 3B is a FESEM image of porous Fe₂O₃/C nanoparticles in accordancewith another embodiment of the present disclosure;

FIG. 3C is a FESEM image of porous Fe₂O₃/C nanoparticles in accordancewith another embodiment of the present disclosure;

FIG. 3D is a FESEM image of porous Fe₂O₃/C nanoparticles in accordancewith another embodiment of the present disclosure;

FIG. 3E is a FESEM image of porous Fe₂O₃/C nanoparticles in accordancewith another embodiment of the present disclosure;

FIG. 3F is a FESEM image of porous Fe₂O₃/C nanoparticles in accordancewith another embodiment of the present disclosure;

FIG. 4A is a schematic illustration of change in composition ofnanoparticles between Fe₂O₃/C to carbon-decorated Fe₃O₄ in accordancewith another embodiment of the present invention;

FIG. 4B is a schematic illustration of change in composition ofnanoparticles between Fe₂O₃/C to carbon-decorated Fe₃O₄ in accordancewith another embodiment of the present invention;

FIG. 5A is an XRD pattern characterizing a carbon-decorated Fe₃O₄nanoparticle;

FIG. 5B is a photograph of carbon-decorated Fe₃O₄ nanoparticles withmagnetic properties in a tube;

FIG. 5C is a FESEM image of carbon-decorated porous Fe₃O₄ nanoparticlesin accordance with another embodiment of the present disclosure;

FIG. 5D is a FESEM image of carbon-decorated porous Fe₃O₄ nanoparticlesin accordance with another embodiment of the present disclosure;

FIG. 5E is a TEM image of carbon-decorated Fe₃O₄ nanoparticles inaccordance with another embodiment of the present disclosure;

FIG. 5F is a TEM image of carbon-decorated Fe₃O₄ nanoparticles inaccordance with another embodiment of the present disclosure;

FIG. 6A is a graphical representation of a thermogravimetric analysis(TGA) profile of carbon-decorated Fe₃O₄ nanoparticles in air to revealthe content of carbon and magnetite;

FIG. 6B is an EDS analysis of carbon-decorated Fe₃O₄ nanoparticles;

FIG. 7A is a graphical representation of an electrochemical measure ofthe performance of an additive-free electrode including carbon-decoratedFe₃O₄ nanoparticles formed by an additive-free centrifugation-assistedmanufacturing process in accordance with another embodiment of thepresent invention;

FIG. 7B is a graphical representation of another electrochemical measureof the performance of an additive-free electrode includingcarbon-decorated Fe₃O₄ nanoparticles formed by an additive-freecentrifugation-assisted manufacturing process in accordance with anotherembodiment of the present invention;

FIG. 7C is a graphical representation of another electrochemical measureof the performance of an additive-free electrode includingcarbon-decorated Fe₃O₄ nanoparticles formed by an additive-freecentrifugation-assisted manufacturing process in accordance with anotherembodiment of the present invention;

FIG. 7D is a graphical representation of another electrochemical measureof the performance of an additive-free electrode includingcarbon-decorated Fe₃O₄ nanoparticles formed by an additive-freecentrifugation-assisted manufacturing process in accordance with anotherembodiment of the present invention;

FIG. 7E is a graphical representation of another electrochemical measureof the performance of an additive-free electrode includingcarbon-decorated Fe₃O₄ nanoparticles formed by an additive-freecentrifugation-assisted manufacturing process in accordance with anotherembodiment of the present invention;

FIG. 8A is a graphical representation of an electrochemical measure ofthe performance of an electrode including carbon-decorated Fe₃O₄nanoparticles formed by a conventional manufacturing process with aninset image of said electrode;

FIG. 8B is a graphical representation of another electrochemical measureof the performance of an electrode including carbon-decorated Fe₃O₄nanoparticles formed by a conventional manufacturing process;

FIG. 8C is a graphical representation of another electrochemical measureof the performance of an electrode including carbon-decorated Fe₃O₄nanoparticles formed by a conventional manufacturing process;

FIG. 9A is a graphical representation of an electrochemical measure ofthe performance of an additive-free electrode including α-Fe₂O₃-carboncomposite nanoparticles formed by a centrifugation-assistedmanufacturing process in accordance with another embodiment of thepresent invention and an inset image of said electrode; and

FIG. 9B is a graphical representation of another electrochemical measureof the performance of an additive-free electrode includingα-Fe₂O₃-carbon composite nanoparticles formed by acentrifugation-assisted manufacturing process in accordance with anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS AND THE PRESENTLY PREFERREDEMBODIMENTS

As used herein with regard to a range, the term “between” is inclusiveof the endpoints of said range, unless it is clear that the endpointsare excluded. For example, when “an integer between 1 and 3” is recited,the integer may have a value of 1, or of 2, or of 3.

As used herein, the terms “substantially” and “about” mean“approximately but not necessarily equal to,” and when used in thecontext of a numerical value or range set forth means a variation of±20%, or less, of the numerical value. For example, a value differing by±20%, ±15%, ±10%, or ±5%, or any value in the range between −20% and+20%, would satisfy the definition of “substantially” or “about.”

In one embodiment, the present disclosure provides a procedure toprepare porous olive-like carbon decorated Fe₃O₄ based additive-freeelectrodes with improved electrochemical performances. The porousFe₂O₃/C nanoparticle with olive-like particles obtained via ultrafast(about 75 minutes) one-pot synthesis was constructed by aggregation ofshort nanorods with a length of about 18 nm and a width of about 8 nm.

A centrifugation-assisted deposition (CAD) method was developed toprepare additive-free carbon decorated Fe₃O₄ where active materials weredirectly prepared on the current collector. In other words, once thesynthesis of active materials finished, the electrodes were ready. Nobinder, conductivity enhancer or solvent was employed. The electrodescan be directly assembled into cells without any post-synthesisprocessing. In contrast, traditional electrode preparation involvesmultiple steps of slurry preparation (mixing), roll-coating,compressing, drying in a vacuum, and additives (for example,polyvinylidene fluoride PVDF binder, carbon black conductivity enhancer,and N-methylpyrrolidone NMP solvent) are employed in the process, inaddition to the conventional procedures to prepare the active electrodematerials. The as-prepared additive-free porous carbon decorated Fe₃O₄based electrodes exhibited superior electrochemical performances inLIBs. A specific capacity of over about 750 mA h g⁻¹, particularly about800 mA h g⁻¹ could be maintained for at least 240 cycles with minimumcapacity fading. Moreover, it exhibited excellent rate performance:specific capacities of 761, 752, and 727 mA h g⁻¹ were achieved at highcurrents of 500, 1000 and 1500 mA g⁻¹, respectively. The results suggestthat the additive-free carbon decorated Fe₃O₄ based electrode obtainedby CAD can be potentially used in high-rate LIBs.

Disclosed herein is a facile procedure to prepare a novel additive-freecarbon-decorated magnetite (Fe₃O₄) electrode with unique nanostructure,inclusive of a substantially ellipsoid or olive-like structure, forapplication in next-generation high-energy lithium-ion batteries. Itincludes an ultrafast solvothermal process to prepare a composite ofporous α-Fe₂O₃ and carbon with a unique olive-like (or substantiallyellipsoid) nanostructure formed by aggregations of orderly alignednanorods as building subunits as the precursor; a novel facilecentrifugation-assisted preparation of additive-free electrode; in situcarbothermic partial reduction of α-Fe₂O₃ to carbon-decorated Fe₃O₄ asadditive-free electrode by a heating (or sintering, or calcination)process in argon; and superior performances in lithium storage foradvanced lithium-ion batteries have been successfully demonstrated.

A fast solvothermal method allows for preparation of porousα-Fe₂O₃-carbon composite with unique olive-like (or substantiallyellipsoid) structures formed by aggregation of orderly-aligned nanorods.The α-Fe₂O₃-carbon nanocomposite can be prepared in a water/alcoholmixture in a reaction time of only 75 minutes, instead of more than 24hours which is normally used in solvothermal/hydrothermal synthesiswidely reported in literature. The as-prepared porous α-Fe₂O₃-carbonnanomaterials can be directly coated on a current collector, such as ametal, in particular a copper disc.

A simple and fast centrifugation-assisted method (CAM) allows forpreparation of additive-free electrodes used for lithium ion batteries.The additive-free α-Fe₂O₃-carbon electrode can be simply prepared in 5min by centrifuging copper disc current collector in ethanol solution ofα-Fe₂O₃-carbon nanomaterials. The as-prepared porous α-Fe₂O₃-carbonnanomaterials can be selectively uniformly coated on one side of thecopper disc current collector directly by the centrifugation of copperdisc in ethanol solution of α-Fe₂O₃-carbon nanomaterials, without anyadditives (e.g. PVDF binder, carbon black conductivity enhancer and NMPsolvent). This represents a contrast to conventional electrodepreparation. The existing traditional technology to prepare electrodeinvolves multiple steps, including slurry preparation, coating,compressing, and drying in vacuum. Also, additives are used to preparethe traditional electrode. Organic polymer additive used as binder ismixed with conductive carbon black additive as conductive enhancer intoxic NMP solvent to coat the as-prepared active material on currentcollector.

The in situ carbothermic partial reduction of α-Fe₂O₃-carbonadditive-free electrode to carbon-decorated Fe₃O₄ additive-freeelectrode is achieved by heating (or calcining, or sintering)α-Fe₂O₃-carbon additive-free electrode in argon in tube furnace. Theα-Fe₂O₃-carbon electrode can be selectively reduced to carbon-decoratedFe₃O₄ electrode by a simple heating (or sintering, or calcination)process with the coating of materials firmly attached on the coppercurrent collector.

The additive-free carbon-decorated magnetite electrodes have superiorelectrochemical performance when compared to those made by conventionalmethods. The specific capacity can be maintained at about greater than750 mAh g⁻¹, or about 800 mAh g⁻¹, after more than 240 cycles at currentof 100 mA g⁻¹. Further, no significant capacity drop is observed whenthe electrode is charged/discharged at a much higher current of about1500 mA g−1 (or about 730 mA h g⁻¹). As compared to the graphitic carbonanode materials in commercial LIBs which have theoretical specificcapacity of 372 mA h g⁻¹, a huge improvement in electrochemicalperformance is realized with these electrodes. The magnetitenanoparticles may be porous nanoparticles.

The ultrafast solvothermal method allows for preparation ofα-Fe₂O₃-carbon nanomaterials in a water-alcohol system. Even though thesolvothermal methods are widely applied to prepare nanomaterials, arelatively long reaction time (e.g. 24 hours or longer) is alwaysrequired to obtain the materials. Here the preparation can be achievedin only about 75 minutes, which shorten the time of preparationsignificantly. This solvothermal process was used here to prepare activematerials in a short time, which is more desirable in industryproduction; prepare α-Fe₂O₃-carbon nanomaterials, precursors ofcarbon-decorated magnetite, which can be directly coated on coppercurrent collector and is essential to prepare the additive-freeelectrode

Contrarily, in a conventional electrode preparation, four steps arerequired to prepare additive-contained electrode, including (1) weighingand mixing of active materials and additives (e.g., PVDF binder, carbonblack conductivity enhancer and NMP solvent), and preparation of slurrythrough grinding the mixture, (2) coating, (3) compressing and (4)drying processes. The polymer binder polyvinylidene fluoride (PVDF) isan adhesive and traditionally used as binder to coat active materials oncurrent collector. However, a relatively long period and vacuum isrequired to dry the binder-contained electrode, normally more than 12hours at 120° C. in vacuum, due to the high boiling point of NMP solvent(202-204° C.). In the process to prepare additive-free carbon-decoratedmagnetite electrode, only two steps are required, including acentrifugation step to deposit of α-Fe₂O₃-carbon nanomaterials oncurrent collector and a heating (or sintering, or calcination) step toconvert α-Fe₂O₃-carbon to carbon-decorated magnetite. The centrifugationand heating (or sintering, or calcination) processes take 5 minutes and220 minutes, respectively. No additives are required, so vacuumconditions and drying time of 12 or more hours is not required, whichsimplifies industrial production and saves money.

The in situ carbothermic partial reduction of α-Fe₂O₃-carbonadditive-free electrode to carbon-decorated Fe₃O₄ additive freeelectrode through a heating (or sintering, or calcination) process inargon: α-Fe₂O₃-carbon additive-free electrodes can be selectivelyreduced to carbon-decorated Fe₃O₄ electrode by a simple heating (orcalcination, or sintering) process in argon, while the copper discs ascurrent collector are stable and inert in the reaction to convertα-Fe₂O₃ to Fe₃O₄. The carbon-decorated Fe₃O₄ was firmly and uniformlyattached on copper current collector to form additive-free electrode anda superior cycling performance and rate performance were achieved inLIBs.

The present disclosure provides for preparation of suitablenanostructured carbon decorated Fe₃O₄ for LIB applications, simplifiedelectrode preparation technology, shortened electrode preparation time,and excellent electrochemical performances in lithium ion batteries.

The procedure to prepare additive-free carbon-decorated magnetite basedelectrodes, including active materials preparation and electrodepreparation, could find applications in lithium ion batteries for energystorage. Making electrodes for lithium ion batteries will in turnproceed at lower cost. Fabrication is simplified and superiorelectrochemical performances are realized. These materials can be usedas anode materials for lithium ion batteries and achieve superiorperformance at reduced cost.

It is rarely reported that the power materials can be directly anduniformly coated on current collector. Here, the ultrafast solvothermalmethod is used to prepare α-Fe₂O₃-carbon nanoparticle, which is in turnused to prepare the additive-free electrode. These α-Fe₂O₃-carbonnanoparticles can be easily coated on copper current collector throughcentrifugation.

Porous nanostructured olive-like (or substantially ellipsoid) carbondecorated Fe₃O₄ (Fe₃O₄/C) was prepared via in situ carbothermaltreatment of the porous olive-like composite of α-Fe₂O₃-carbon. Theprecursor porous olive-like (or substantially ellipsoid) α-Fe₂O₃-carbonnanoparticles are prepared through an ultrafast (75 minutes) one-potsolvothermal method. The porous olive-like Fe₃O₄/C nanostructures areformed by joined (interconnected) nanorods, and its precursorα-Fe₂O₃-carbon nanostructures, are formed by aggregation oforderly-aligned nanorods as building subunits. The successful conversionfrom α-Fe₂O₃-carbon to Fe₃O₄/C by in situ carbothermic partial reductionis evidenced by extensive characterization, as shown in the drawings andin the description below.

The porous olive-like α-Fe₂O₃-carbon nanoparticles dispersed in ethanolare coated on Cu current collectors by centrifugation-assisteddeposition (CAD) directly without any additives employed andsubsequently heated to convert to a Fe₃O₄/C coated current collector,such as a copper disc, which can be used as an electrode without anyfurther processing. In contrast, traditional electrode preparationinvolves multiple steps of slurry preparation, coating, compressing,drying in a vacuum and additives (e.g., PVDF binder, carbon blackconductivity enhancer and NMP solvent) are employed in the process.

The as-prepared additive-free, porous, Fe₃O₄/C based electrodeconstructed from nanoparticles having both iron and carbon within thenanoparticle exhibited superior electrochemical performances in lithiumstorage. Excellent cycling performance was achieved with a specificcapacity at >800 mA h g⁻¹ for at least 240 cycles. Impressive rateperformance was accomplished when tested under different currents: nosignificant capacity drop was observed when current was increased from100 mA g⁻¹ to 1500 mA g⁻¹ and a specific capacity of 730 mA h g⁻¹ couldbe maintained at current of 1500 mA g⁻¹. The electrode in thisembodiment did not include a magnetite nanoparticle that was coated withcarbon, but rather a nanoparticle that contained both magnetite andcarbon in a single particle.

In a typical synthesis, an iron source is combined with a carbon sourcein an aqueous solution to define an iron-carbon solution. In oneembodiment, about 0.4 mmol of FeCl₃.6H₂O and about 0.4 mmol of D-glucosewere fully dissolved in 16 ml of deionized water. Many different ironcompounds may be used, as may many different carbon sources.Particularly useful are iron halides, such as iron fluorides, ironchlorides, iron bromides, and so forth.

Continuing with the exemplary embodiment, a porosity-increasing andstructure-controlling compound is added. Without such a compound, thestructure of the nanoparticle is solid, and has a different morphology(that is, the particle is no longer olive-like in structure.) In oneembodiment, the porosity-increasing compound may be an anion or provideanions to the mixture. In one embodiment, the anion has a charge of −2.In one embodiment, the porosity-increasing compound is dimethyl oxalate.In the exemplary embodiment, about 0.4 mmol of dimethyl oxalate wasfully dissolved in about 16 milliliters (ml) of 1-propanol. The formersolution was added drop by drop to the later solution. The mixture wasstirred for about 15 minutes, and then transferred into a 45 mlTeflon-lined autoclave and heated at 200° C. for about 75 minutes.Further variations may be used, such as a heating temperature range ofabout 160° C. to about 225° C., and a time of about 45 minutes to about120 minutes. The solid products are collected and washed repeatedly withwater and ethanol, and dried at about 80° C. in a vacuum oven overnight.This gives rise to a collection of olive-like (or substantiallyellipsoid) iron-carbon nanoparticles.

The process of generating an electrode using said nanoparticles isillustrated in FIG. 1F, and in FIGS. 1A-1B. In one embodiment, acalculated amount of olive-like Fe₂O₃/C powders is dispersed into about5 ml of ethanol in a 50 ml centrifuge tube under ultrasonication toachieve a highly homogeneous suspension. A current collector, such asfor example a piece of clean copper disc typically used as a currentcollector, is placed into the centrifuge tube and was centrifugedtogether with the mixture solution at about 6000 rpm for about 5 min.The olive-like (or substantially ellipsoid) α-Fe₂O₃-carbon nanoparticleswere deposited by centrifugal force and coated firmly on only one sideof the copper disc (see for example FIG. 1D, middle image). Centrifugingin this manner selectively coats a single face of the current collector(see FIG. 1E) which is a desirable outcome. The olive-like Fe₂O₃-carboncoated copper disc is dried in a vacuum oven at 80° C. overnight, whichcould be further modified to a significantly shorter time due to theeasy evaporation of ethanol solvent. To prepare an olive-like (orsubstantially ellipsoid) carbon decorated Fe₃O₄ based electrode, thecoated copper disc was heated (or sintered, or calcined) in a tubefurnace at a heating rate of about 6° C. per minute to a finaltemperature of about 600° C. and maintained at a predetermined sinteringtemperature for a predetermined time of about 2 hours under argon (seeFIG. 1C, FIG. 4A-4B).

Particles made according to the above protocol and generally inaccordance with the principles of the present disclosure stick to a widevariety of surfaces: metals, glass, plastics, and so forth. Withoutwishing to be bound by any particular theory, this intrinsic stickinessmay be due to functional groups on the carbons or the iron of thenanoparticle, or from decomposition of glucose, or the nanostructurethat makes up the particle. Theory notwithstanding, this property of thecomposition is notable, as it allows for binding between thenanoparticle and the current collector with no binder, and no additive.Stated another way, the electrode is made by direct binding (orattachment) of the nanoparticle to a face of a current collector. Thisdirect binding is made possible by properties particular to thenanoparticles disclosed herein, and is distinct from protocols andassemblies wherein a binder is used to affix nanoparticles to a surface.The elimination of binder can allow for a greater quantity, in someembodiments as measured by surface area, in other embodiments asmeasured by volume, of the active material which is made up ofnanoparticles, rather than some of this surface area or volume beingoccupied by the binder or another additive.

Without being bound by any theory, a porous nanoparticle, rather than asolid nanoparticle, may have certain advantages. For instance, theporosity may increase infiltration of the electrolyte of a LIB into theelectrode, providing greater interaction between these two components.

Furthermore, nanoparticles formed in this way constitute a type ofnanoparticle which has both carbon and iron present at its surface. Thisstands in contrast to other types of nanoparticles which may be made ofan iron-containing compound and which are coated with a layer of acarbon-containing compound after formation of the iron-basednanoparticle. As will be described below, these noncoated iron-carbonnanoparticles can be used in the construction of electrodes which haveadvantageous properties.

Powder X-ray diffraction (XRD) was carried out with a Rigaku SmartlabX-ray diffractometer using Cu K^(α) radiation (λ=0.15418 nm). Themorphology of the materials was characterized by field emission scanningelectron microscopy (JSM-7600 FE SEM, with an accelerating voltage of 15kV) and by trans-mission electron microscopy (JEOL 2010 TEM instrument,with an accelerating voltage of 200 kV). See FIG. 2A. Theenergy-dispersive X-ray spectroscopy (EDS) was measured using PegasusApex 2 integrated EDS. See FIG. 2G. Thermal gravimetric analysis (TGA)was carried out with SDT Q600 (TA Instruments) under a dynamic airatmosphere with a ramping rate of 10° C. min⁻¹ to 900° C. See FIG. 6A,which shows TGA results of olive-like carbon-coated Fe₃O₄ in air.

The additive-free electrodes of olive-like (or substantially ellipsoid)carbon decorated Fe₃O₄ coated on a copper disc as obtained by CAP wereassembled into testing cells directly. Electrochemical test cells wereassembled in an argon-filled glove box using the additive-free electrodeas the working electrode, metallic lithium foil as the counter andreference electrode, 1 Molar (M) solution of LiPF₆ in a 50:50 v/vmixture of ethylene carbonate (EC) and diethyl carbonate (DEC) as theelectrolyte, and PP/PE/PP trilayer membranes (Celgard 2320) as theseparator. The cells were charged and discharged galvanostatically atroom temperature in the voltage window of 0.01-3 V at different currentson a MTI BST8-WA battery tester.

The overall preparation process of porous olive-like α-Fe₂O₃-carbonnanoparticles as precursors was achieved by 75 min of solvothermalreaction, in one embodiment in a sealed Teflon reactor in stainlesssteel autoclave. The as-obtained products are shown in FIGS. 2B and 2C.Porous olive-like carbon decorated Fe₃O₄ nanoparticles with roughsurfaces were obtained by heating (or sintering, or calcining) theporous olive-like α-Fe₂O₃-carbon precursor under the protection ofargon. Without wishing to be bound by any particular theory, the carboninside the olive-like Fe₂O₃-carbon nanoparticles likely played a role topartially reduce Fe₂O₃ to Fe₃O₄ during heating (or sintering, orcalcination) of porous olive-like α-Fe₂O₃-carbon due to carbothermicreactions.

Porous olive-like Fe₂O₃-carbon nanoparticles, the precursor for makingcarbon decorated Fe₃O₄, were prepared through a fast solvothermalreaction with a reaction time of only 75 min and characterized by XRD,FESEM and TEM (FIG. 2A-2F, 3A-3F). The XRD pattern of the as-preparednanoparticle can be indexed to α-Fe₂O₃ with rhombohedral structure(JCPDS card no. 33-0664) and no impurities are detected in the pattern(FIG. 2A). The crystalline size calculated from the Scherrer equationbased on (110) peaks is 18 nm, which is in accordance with the size ofbuilding subunits of the porous nanoparticles observed under FESEM (FIG.2D) and TEM (FIG. 2F). The absence of carbon peaks suggests that thecarbon from hydrothermally carbonized glucose is mainly amorphous. Thecarbon of the nanoparticles of the present disclosure is dispersed;there is not a large clustering which includes high-carbon regions asthough the composite had been prepared by a different method, such as byinclusion of carbon quantum dots.

The EDS (FIG. 2G) exhibits that the as-prepared particles contain theelements of carbon, iron and oxygen as expected. Carbon was producedthrough the carbonization of glucose during the solvothermal processabove 160° C. The typical morphology and structure of the olive-like (orsubstantially ellipsoid) Fe₂O₃-carbon nanoparticles are clearly revealedby the FESEM images (FIG. 2B-2D). The overall morphology and sizedistribution of the olive-like Fe₂O₃-carbon nanoparticles are revealedby the low-magnification FESEM image (FIG. 2B). The aspect ratio of theolive-like structure is around 3:2, as shown in the high-magnificationimage (FIG. 2C).

The olive-like α-Fe₂O₃-carbon nanoparticles are about 300-600 nm inlength and about 200-400 nm in diameter. More details of the internalstructure of the nanoparticle are shown in the zoom-in view FESEM image(FIG. 2D): the olive-like nanoparticle is formed by aggregation ofordered small nanorods with a length of about 18 nm and a width of about8 nm (which was further confirmed by TEM). A broken olive-like structureat the center of the high-magnification FESEM image shows that the wholestructure is highly porous, from the core to the surface. The structureof the α-Fe₂O₃-carbon composite is further revealed by the TEMcharacterization (FIG. 2E and FIG. 2F).

A set of experiments were carried out to study the effect of reactiontimes on the structure of the olive-like Fe₂O₃-carbon precursors. Thosenanoparticles obtained at different reaction times were characterized byFESEM and TEM (FIG. 3A-3F). When the reaction time was about 45 minutes,less porous olive-like nanoparticles were formed (FIGS. 3A-3B, and theinset of 3B). The nanorod subunits, with the length and width of around18 and about 8 nm, respectively, were aggregated to form the olive-likestructure and closely packed (FIG. 3B). All the subunits of nanorods arewell aligned in the same way, radiating from the core to the outside, asshown in FIG. 3B. As the reaction time increased from 45 minutes to 75minutes, the olive-like structure became more porous, as evidenced bythe more distinguishable gaps between fat nanorod subunits in FIGS. 3Cand 3D. The inset of FIG. 3D reveals that the whole olive-like structureis porous, which is consistent with the broken olive-like structureobserved in the high magnification FESEM image (FIG. 2D).

As the reaction time further increased to 3 hours, the porous nature waspreserved, as shown in FIG. 3E and the inset of FIG. 3F. However, with alonger reaction time, the length and diameter of the nanorod subunitsincrease to about 23 nm and about 10 nm, respectively. Also, theorientations of the subunits become more random, as compared to thesamples prepared with shorter reaction times of 45 minutes and about 75minutes. Besides the reaction time, the porosity and the shape of theas-prepared olive-like structure can be affected by the amount ofglucose. With 0.4 mmol or 0.2 mmol glucose, the olive-like (orsubstantially ellipsoid) structures are highly porous with the nanorodsas the basic building units. Occasionally, by-products of hollownanoparticles were also observed. For comparison, with only 0.1 mmolglucose, the porosity of the olive-like structure is less than thoseobtained with 0.4 mmol or 0.2 mmol of glucose. A tunnel which connectsthe two ends of the olive-like structure and two open ends, instead ofthe highly porous structure was observed. We observed that the aspectratio of the as prepared olive-like nanoparticles could be tuned throughthe control of the amount of glucose. The aspect ratio increases fromabout 3:2 to about 2:1 when the amount of glucose added is decreasedfrom 0.4 mmol to 0.1 mmol. Without any glucose, a cocoon-like hollowstructure with an aspect ratio of about 2.5:1 could be achieved.

Based on the experimental observation, a plausible formation mechanismis proposed. The formation of porous olive-like nanoparticles startsfrom the hydrolysis of Fe³⁺ ions and nucleation in solution and thesubsequent formation of nanorods. The nanorods as building unitsaggregate to form an olive-like structure and the nanorods are denselypacked initially (FIG. 3A). The highly porous olive-like aggregates formwith a longer reaction time (FIG. 3B). The small nanorod subunits, whichpossess higher surface energy in solution, tend to be dissolved moreeasily to minimize the total surface energy. Thus, the big nanorods growwith the continuous dissolving of small subunits and re-deposition onlarge subunits. With the decrease of the number of small nanorods andthe increase of the size of the large nanorods, void spaces aregenerated and the particles become porous. Meanwhile, the aspect ratioand the porosity can be tuned through the control of glucose. It mightbe because the glucose or the deposited carbon may influence the surfaceproperty of the nanorod subunits and their aggregation. Thus, theorientated alignment of the subunits could be different from those inthe system without glucose.

The as-prepared olive-like Fe₂O₃-carbon could be converted to carbondecorated Fe₃O₄ preserving the overall morphology well. The successfulpartial reduction of Fe₂O₃ was proved by the evidence that Fe₃O₄ wasproduced as revealed by XRD (FIG. 5A). All the peaks of the XRD patterncan be assigned to Fe₃O₄ (JCPDS card no. 75-0449), and no peak ofa-Fe₂O₃ is observed, suggesting that carbothermic conversion of Fe₂O₃into Fe₃O₄ was successful and complete. The crystalline sizes of asprepared Fe₃O₄ are about 45 nm, calculated based on the (220) peak,which is significantly larger than the crystallite size of the Fe₂O₃nanoparticle at around 20 nm. The typical black color of Fe₃O₄ ratherthan the typical red color of Fe₂O₃, was obtained and all the blackpower of olive-like carbon decorated Fe₃O₄ attracted by a magnetic bar(FIG. 5B) demonstrate that Fe₂O₃ was successfully converted to Fe₃O₄.Note that the powder of carbon decorated Fe₃O₄ was obtained withoutbeing coated on the copper disc by CAP in this case and the nanoparticlepowder was heat treated similarly under argon in a crucible. Theconversion from Fe₂O₃ to Fe₃O₄ under inert gas also proves the existenceof carbon produced by the solvothermal carbonization of glucose, whichacts as the only reducing agent to convert Fe₂O₃ to Fe₃O₄.

The morphology of as-synthesized olive-like carbon decorated Fe₃O₄ wasrevealed by FESEM and TEM images in FIG. 5C-5F. After the heating to (orsintering at or calcination at) 600° C. under argon, the olive-like (orsubstantially ellipsoid) structure was maintained (FIG. 5C and FIG. 5D).The porosity of the olive-like carbon decorated Fe₃O₄ is also preserved,revealed by the broken olive-like structure in FIG. 5D. The size ofsubunits of carbon decorated Fe₃O₄ increased to around 50 nm from about18 nm subunits in the Fe₂O₃-carbon nanoparticle (FIG. 5E and FIG. 5F),which is consistent with the sizes estimated from the XRD. The increasein size of subunits can be attributed to the high temperature annealinginduced crystalline growth.

As compared to the Fe₂O₃-carbon aggregates assembled by alignednanorods, carbon decorated Fe₃O₄ was constructed by joined subunits. Theconnection of building subunits may be attributed to the annealingprocess where subunits are joined by grain boundary diffusions which wasalso observed in the study of other metal oxide nanomaterials. It mayalso be attributed to the consumption of carbon in Fe₂O₃-carbon throughthe carbothermic reduction process which eliminates the impuritiesbetween grain boundaries. The SAED pattern (the inset of FIG. 5F)demonstrates that the olive-like carbon-decorated Fe₃O₄ is crystallinewith all the diffraction spots assigned to Fe₃O₄.

The carbon content in the as-prepared Fe₃O₄/C materials isquantitatively determined by the TGA analysis (FIG. 6A). The smallweight loss below 150° C. is due to the evaporation of the adsorbedmoisture or gaseous molecules. The combustion of carbon begins at around300° C. Fe₃O₄ can be oxidized to Fe₂O₃ when heated in air, and thetheoretical weight increase is 3.45%, based on the chemical reaction4Fe₃O₄+O₂→6Fe₂O₃. From TGA analysis, the carbon content of the carbondecorated Fe₃O₄ nanoparticle was estimated to be 0.43%.

The as-prepared olive-like carbon decorated Fe₃O₄ nanoparticlefabricated directly on a current collector (in this case, a copper disc)was used as an additive-free ready electrode and assembled into acoin-type cell directly and evaluated. The electrochemical properties ofthe as-prepared additive-free Fe₃O₄/C electrode are shown in FIG. 7. Anoptical image of the additive-free carbon decorated Fe₃O₄ as a readyelectrode for cell assembly is shown in the inset of FIG. 7A. Thecharge-discharge profiles of the first two cycles at the current of 100mA g⁻¹ with a cutoff voltage window of 0.01-3.0 volts (V) are shown inFIG. 7A. In the first discharge curve, two potential plateaus at about0.9 and 0.8 V are observed, which can be ascribed to the formation of aLi—Fe—O compound, the conversion reaction of the Li—Fe—O compound to Fe⁰and the formation of the Li₂O matrix. This profile is different from thefirst cycle discharge profile of Fe₂O₃ negative-electrode materials,which has two potential plateaus at around 1.6 volts and 0.8 V versusLi+/Li (FIG. 9A). For the Fe₂O₃/C precursor, the potential plateau ataround 1.6 V can be ascribed to the intercalation of Li⁺ into α-Fe₂O₃,and the other potential plateau at around 0.8 V is due to the formationof Li₂(Fe₂O₃) and the reduction of Fe ions to Fe⁰. The differencebetween the first cycle discharge profile of carbon decorated Fe₃O₄(FIG. 7A) and the Fe₂O₃/C precursor materials (FIG. 9A) illustrates thedifferent electro-chemical reactions involved during the first cycle ofLi insertion, which also indirectly proves the successful conversionfrom Fe₂O₃ to Fe₃O₄ by in situ partial carbothermic reduction. Thevoltage drop from below 0.8 to 0.01 V could be attributed to theformation of a solid electrolyte interphase (SEI) due to thedecomposition of the solvent in the electrolyte.

In the first cycle charge profile, the plateau around 1.7 V isattributed to the oxidation of Fe⁰ to Fe²⁺ and Fe³⁺. The first cycledischarge and charge capacities are 1579 and 917 mA h g⁻¹, respectively.The irreversible capacity loss of around 42% could be attributed to thedecomposition of the electrolyte and formation of the solid electrolyteinterphase (SEI). To better interpret the electrochemical reactionsinvolved, the dQ/dV vs. V plots for the first two cycles are shown inFIG. 7B, which match well with plateaus discussed above. Additionally,the absence of the typical reduction peak at around 1.6 V widelyobserved for Fe₂O₃ proves that the electrode is based on Fe₃O₄. In the2nd cycle, both reduction and oxidation peaks are positively shifted ascompared to the 1st cycle.

The charge-discharge voltage profiles at different currents of 100, 200,500, 1000 and 1500 mA g⁻¹ are shown in FIG. 7C, which are at the 2nd,12th, 22nd, 32nd, and 42nd cycle, respectively. The charge-dischargevoltage profiles at different currents almost overlap, whichdemonstrates the same reversible electro-chemical reactions involved.The specific discharge capacities are 919, 772, 761, 752, and 727 mA hg⁻¹ at currents of 100, 200, 500, 1000 and 1500 mA g⁻¹, respectively. Aswe can see, the capacity fades slowly at the currents from 200 to 1500mA g⁻¹, which has been rarely observed for metal oxides tested in LIBs.And even at a high current of 1500 mA g⁻¹, the specific dischargecapacities can still be maintained at 727 mA h g⁻¹, which shows improvedrate performance as-compared to most reported Fe₃O₄ nanomaterial basedanodes.

The rate performances of the additive-free Fe₃O₄/C electrode wasevaluated by charging-discharging the battery cells at different currentdensities for 10 cycles interval (FIG. 7D). The average Coulombicefficiency from the 2nd cycle is as high as 98.92%, which indicates theexcellent electrochemical reversibility. The capacities are very stableand no distinguishable capacity drop can be observed at each current. Itis interesting to highlight that no significant capacity drop wasobserved when current was increased from 100 mA g⁻¹ to 200, 500, 1000,and 1500 mA g⁻¹, which suggests that the electrodes could be used infast charge LIBs. It is interesting to observe that the specificcapacity was recovered to 835.5 mA h g⁻¹ after 60 cycles, when thecurrent changed back to 100 mA g⁻¹. More interestingly, we observe anincrease of capacity after the current is recovered to 100 mA g⁻¹ from1500 mA g⁻¹. This could be explained by the continuous activatingprocess of the electrode. Excellent cycling performance was achievedwith a specific capacity at >about 800 mA h g⁻¹ for at least about 240cycles (FIG. 7E). The superior electrochemical performances could beattributed to its porous structure induced good accessibility of Li⁺ions and improved strain accommodation and the way electrode wasprepared. In contrast, the conventional electrode prepared from the sameFe₃O₄/C demonstrated poor cycling relative to the novel electrode (FIG.8A-8C), which indicates the impact of the role of electrode preparation.Thus, the porosity, the additive-free nature, the aggregation of alignednanorods, and the substantially ellipsoid shape may all play roles inimproving electrode properties over solid nanoparticles. The olive-likeshape could still be observed from the sample of Fe₃O₄/C nanoparticlesafter cycling, indicating good stability. As a comparison, theadditive-free Fe₂O₃/C electrode shows very poor electrochemicalperformance (FIG. 9A-9B).

Apart from being used in electrodes for energy storage, porousolive-like carbon decorated Fe₃O₄ and porous olive-like Fe₂O₃/Cprecursor nanoparticles in accordance with the principles of the presentdisclosure may find use as catalysts, drug delivery, water-treatment,and sensors.

In summary, porous olive-like carbon decorated Fe₃O₄ was successfullyprepared from porous olive-like Fe₂O₃/C precursor nanoparticles. Theprecursor was obtained by a fast one-pot solvothermal method. The activematerials were synthesized on a current collector by CAD directlywithout any binder, conductivity enhancer or solvent employed. Whentested as negative-electrode materials for LIBs, the additive-freeolive-like Fe₃O₄/C electrodes exhibited excellent electrochemicalperformances for lithium storage demonstrating their promising potentialto be carbon alternatives for LIBs.

The invention claimed is:
 1. An electrode for a lithium ion battery, theelectrode comprising: a metal current collector having a first face anda second face opposite the first face; and a plurality of nanoparticleshaving a particulate form and comprising a composite of Fe₃O₄ andcarbon, the nanoparticles being porous and having a substantiallyelliptical structure, the nanoparticles being bound directly to thefirst face of the metal current collector without a polymeric adhesive,the nanoparticles having a carbon content by weight of less than 4%,wherein the electrode has a specific capacity of at least about 750mAh/g after about 240 charge-discharge cycles.
 2. The electrodeaccording to claim 1 wherein the nanoparticle has carbon dispersedthrough the nanoparticle.
 3. The electrode according to claim 1, havinga specific capacity of at least about 700 mAh/g when exposed to acurrent of about 1500 mA/g.
 4. The electrode of claim 1, wherein thenanoparticle is about 300 to about 600 nanometers in length, and about200 to about 400 nanometers in diameter.
 5. The electrode of claim 1,wherein the carbon content by weight of the nanoparticle is less than1%.
 6. The electrode of claim 1, wherein α-Fe₂O₃ is not detected in thenanoparticle by x-ray diffraction.
 7. The electrode of claim 1, whereina length of the nanoparticle is greater than a diameter in a directionnormal the length.